Hybrid circuits, or SLICs, are known for providing signal conversion between a balanced two-wire, bidirectional subscriber loop and a pair of unidirectional transmission paths, the latter comprising an incoming signal path and an outgoing signal path. Typically, SLICs are employed in telephone communication systems between the central office equipment and the telephone hand set found in the subscriber loop as is understood.
U.S. Pat. No. 4,004,109 discloses such a hybrid circuit which comprises a plurality of current mirror circuits of complementary type which provides interface between the aforementioned transmission paths. In addition, U.S. Pat. No. 4,300,023, filed Aug. 13, 1979, entitled "Hybrid Circuit" to Kelley et al and assigned to Motorola Inc. describes a SLIC suitable for manufacture in integrated circuit form and further shows a loop sensing circuit for coupling between the SLIC and the bidirectional subscriber loop.
As explained later in greater detail, the loop sensing circuit includes a first pair of resistors serially coupled in a path to one loop terminal of the subscriber loop and a second pair of serially connected resistors coupled in the path to the other loop terminal. These paired resistors each include a protection resistor and a sensing resistor as is known and are coupled to particular terminals of the SLIC such that the loop sensing circuit provides impedance matching to the subscriber loop as well as protection to the monolithic SLIC against high voltage transients. The loop sensing circuit in combination with the SLIC provides longitudinal signal balanced to suppress longitudinal signals that may occur at the subscriber loop terminals.
As disclosed in the aforementioned Kelley et al patent, dc resistance matching between the SLIC and the subscriber loop is provided by paralleling the particular sensing resistor (R1 or R2) of each pair of serially connected resistor paths by a high current gain loop to effectively reduce the high resistive value of the sense resistor (approximately 16,000 ohms) to approximately 170 ohms which, in series with its respective protection resistor (R3 or R4) which has a value of 30 ohms, presents a balanced 400 ohm resistance, 200 ohms to each loop terminal, to the subscriber loop as generally is required. Because the value of each protection resistor is a large percentage of the dc resistance appearing at each loop terminals, it has been found that to maintain good longitudinal balance for suppressing longitudinal signals (while providing protection to the SLIC to secondary lightning strikes) the two protection resistors must be matched within approximately 0.1% of each other in value. Thus, the cost of these two protection resistors becomes substantial because of this matching tolerance along with the need for these resistors to dissipate large power transients due to proximate lightning strikes. Moreover, signal sensitivity is somewhat degraded between the subscriber loop and the SLIC of the prior art because of the serial connection of the two resistor paths.
Thus, a need exists for an improved loop sensing circuit for improving signal sensitivity while maintaining the system longitudinal signal suppression and reducing system costs.